Despite the development of recent medical imaging modalities, such as computed tomography (CT), ultrasound, nuclear medicine and magnetic resonance imaging (MRI), all of which are digital, X-ray imaging systems remain an important tool for medical diagnosis. Although the majority of X-ray imaging systems in current use are of analog design, digital radiology is an area of considerable recent growth. Digital radiology provides significant advantages over its analog counter-part, such as: easy comparison of radiological images with those obtained from other imaging modalities; the ability to provide image networking within a hospital for remote access and archiving; facilitating computer aided diagnosis by radiologists; and facilitating teleradiology (ie. remote diagnostic service to poorly populated regions from a central facility).
There are currently two commercial approaches to digital radiography--(1) the digitization of a signal from a video camera optically coupled to a an X-ray imaging intensifier, and (2) stimulable phosphor systems. Prior art intensifier systems permit instant readout whereas prior art stimulable phosphor systems require the operator to carry a cassette to a reader. Neither of these systems provide image quality which is acceptable for all applications.
Digital systems based on the use of X-ray image intensifiers suffer from the following disadvantages: the bulky nature of the intensifier often impedes the clinician by limiting access to the patient and prevents the acquisition of important radiographic views; loss of image contrast due to X-ray and light scattering (i.e. veiling glare); and geometric (pin cushion) distortion on the image due principally to the curved input phosphor.
Another prior art X-ray imaging modality which is currently experiencing renewed interest, is the use of amorphous selenium photoconductors as an alternative to phosphors. Xeroradiography, (i.e. the use of amorphous selenium (a-Se) plates which are read out with toner), was a technical and commercial success in the early 1970's. Xeroradiography is no longer commercially competitive. This is believed to be because of the toner readout method, and not because of the underlying properties of a-Se. Commercial as well as scientific interest in a-Se has recently revived. For example, Philips has announced the commercial availability of an a-Se drum scanner for chest radiography based on earlier work at its research laboratories in Aachen. Kodak uses an a-Se plate readout with a phosphor coated toner and laser scanner for the preparation of highly detailed mammography images which are free from significant artifacts. 3M have also published preliminary descriptions of their work on laser discharge readout of a-Se. This work is related to much earlier publications by (1) Korn et al, "A method of electronic readout of electrophotographic and electroradiographic images", Journal of Applied Photographic Engineering, 4, 178-182 (1978); (2) Zermeno et al "Laser readout of electrostatic images", In: Application of Optical Instrumentation to Medicine VII, Edited by J. Gray, et al, SPIE 173, 81-87 (1979); and (3) DeMonts et al, "A new photoconductor imaging system for digital radiography", Medical Physics, 16, 105-109 (1989).
The basis of all existing medical X-ray imaging systems is a phosphor layer or "screen". X-rays absorbed by the screen release light which must reach the surface to create an image. The lateral spread of light is limited only by diffusion and hence is related to the thickness of the screen. Thus, the thicker the screen (which is desirable to increase the quantum absorption efficiency), the more blurry the image will be. This represents a loss of high frequency image information in prior art phosphor systems which is fundamental and largely irreversible. This loss can be alleviated to some extent by using a phosphor such as CsI which can be grown in the form of a fibre optic.
A better method has been discovered for eliminating blurring, which involves using a structureless photoconductor to detect X-rays. X-rays interacting in the photoconductor release electron-hole pairs which are drawn directly to the surfaces of the photoconductor by an applied electric field. The latent charge image on the photoconductor surface is therefore not blurred significantly even if the photoconductor layer is made thick enough to absorb most incident X-rays. Amorphous selenium (a-Se) is the most highly developed photoconductor for X-ray applications. Its amorphous state maintains uniform characteristics to very fine levels over large areas. A large area detector is essential in radiography since no means are provided to focus the X-rays, thereby necessitating a shadow X-ray image which is larger than the body part to be imaged.
One area of intense research in the field of photoconductor X-ray detectors, is the development of systems for charge readout. Antonuk et al disclosed the concept of an X-ray imaging detector which utilizes active matrix arrays for charge readout, as described in the following publications: (1) "Signal, noise, and readout considerations in the development of amorphous silicon photodiode arrays for radiotherapy and diagnostic imaging", Medical Imaging V: Imaging Physics, SPIE 1443, 108-119 (1991), (2) "High resolution, high frame rate, flat panel TFT arrays for digital X-ray imaging", Medical Imaging 1994: Physics of Medical Imaging, Rodney Shaw, Editor, Proceedings of SPIE, 2163, 118-128 (1994) and (3) "Demonstration of megavoltage and diagnostic X-ray imaging with hydrogenated amorphous silicon arrays", Medical Physics 19, 1455-1466 (1992). Their initial research has subsequently been developed by others: Ichiro Fujieda, Robert A. Street, Richard L. Weisfield, Steve Nelson, Per Nylen, Victor Perez-Mendez and Gyuseong Cho, "High sensitivity readout of 2d a-Si image sensors", Jpn. J. Appl. Phys. 32, 198-204 (1993); Henri Rougeot, "Direct X-ray photoconversion processes", In: Digital imaging: AAPM 1993 Summer School Proceedings Ed: William Hendee and Jon Trueblood (AAPM monograph 22, Medical Physics Publishing, 1993) pp. 49-96; UW Schiebel, N Conrads, N Jung, M Weilbrecht, H Wieczorek, T T Zaengel, M J Powell, I D French and C Glasse "Fluoroscopic X-ray imaging with amorphous silicon thin-film arrays", Medical Imaging 1994: Physics of Medical Imaging, Rodney Shaw, Editor, Proc. SPIE, 2163, 129-140 (1994); and M J Powell, I D French, J R Hughes, N C Bird, O S Davies C Glasse and J E Curran, "Amorphous silicon image sensor arrays", Mat. Res. Soc. Symp. Proc. 258, 1127-1137 (1992).
In these prior art systems a phosphor screen (preferably a structured CsI layer) is used to absorb X-rays, and the resultant light photons are detected by an active matrix array with a single photodiode and transistor at each pixel. Antonuk coined the acronym MASDA for "Multi-element Amorphous Silicon Detector Array".